One hundred years ago, you wouldn't be having a good day near the Podkamennaya Tunguska River in what is now Russia's Krasnoyarsk region. That's because on June 30, 1908, this remote tract of mosquito-infested taiga endured the largest cosmic impact of the Industrial Age.

The Tunguska event didn't leave a crater — in fact, to this day investigators have never recovered a trace of the impactor itself — but its calling card was unmistakable. The interplanetary intruder came from the southeast, cutting a white-hot swath across the sky before exploding a few miles above the ground with a thousand times more destructive force than the nuclear blast that leveled Hiroshima, Japan.

The resulting shock wave toppled trees across more than 800 square miles, felling them in a radial pattern that pointed the way back to ground zero. A fireball incinerated anything closer in, including more than a thousand reindeer. Yet, owing to the area's remoteness, only one nomad lost his life.

Tunguska also left its mark throughout the Eastern Hemisphere. A seismograph in St. Petersburg, 2,500 miles to the west, recorded tremors in the ground. The night sky, lit up with tons of stratospheric dust, remained bright enough to cast shadows. Halos were common around the Sun.

Yet scientists of the day weren't interested in getting answers for these puzzling observations, and the Soviet Academy of Sciences didn't dispatch an expedition to the site until 1927. Its leader, Leonid Kulik, was amazed to find a sea of felled trees stretching to the horizon. Realizing that no terrestrial event could have caused devastation on this scale, he sought in vain for the large meteorite he believed to lie beneath the bogs at the blast's epicenter.

Over the years, all kinds of extraordinary causes have been invoked to explain the Tunguska event: a titanic explosion of subterranean methane, nuclear fusion from the deuterium in a comet's ices, collisions of Earth with a small black hole or antimatter — and, of course, the crash of an alien spacecraft.

Today impact specialists have ruled out all of these scenarios, but they remain puzzled by many of the circumstances surrounding the event. Last year an Italian-led expedition found evidence that a nearby lake might be the impact site. And theoretical modeling argues that the blast was much less energetic than originally thought — perhaps equivalent to "just" 5 megatons of TNT.

Whatever the particulars, impact specialists now realize that Tunguska was a wake-up call to the scale of devastation that even a small asteroid or comet could wreak. And cosmic oddsmakers now predict that collisions of this magnitude occur every 1,000 years or so. So, in some sense, we're lucky that Tunguska happened so recently. Scientists have been able to assess the devastation firsthand and ponder how to avoid a "next time."

To mark this remarkable anniversary, Sky & Telescope asked a select group of solar-system specialists to reflect on Tunguska's legacy. Here's what they had to say: